1. Field of the Invention
The described apparatus and methods relate to detecting energetic neutrals of an ion beam. More particularly, the described apparatus and methods are directed for detecting neutrals of an ion beam in an ion implanter to minimize and prevent energy contamination of a workpiece.
2. Related Art
Deceleration of an ion beam reasonably close to a workpiece, such as a wafer, is a standard method of improving ion implanter productivity for low energy beams. The main benefit of deceleration is to reduce the distance the beam must travel at low energy where the efficiency of transporting the beam is poor. The closer the deceleration is to the workpiece, the more benefits result as far as increasing the beam current. However, ions that become neutral prior to the deceleration, but within a line of sight of the wafer, will be implanted at their undecelerated energy and are classified as energy contamination. FIG. 1 illustrates how energy contamination may occur in an ion implanter. As an ion beam 10 propagates through the implanter, a bend magnet 12 may direct beam 10 toward drift space 14. In drift space 14, collisions with surfaces and background gas produce energetic neutrals. Through a deceleration stage 16, ions are decelerated to a final energy in one portion 18 of beam 10. A small flux of the higher energy neutrals remain as a second portion 20 of beam 10. The neutrals that pass to a wafer 22 will implant significantly deeper than the ions to cause energy contamination. Only a small amount of energy contaminated ions are allowed to be implanted, typically on the order of 0.2% to 0.5%, before the implantation of the workpiece is adversely effected.
Known techniques for limiting energy contamination include an implanter architecture where an electrostatic or magnetic bend is placed between the deceleration stage and the magnet, increased pumping to limit the neutralization of beam ions by residual gas, an aperture and liner design to prevent neutrals formed by collisions with the structures inside the implanter from reaching the workpiece, and limiting the voltage allowed when running deceleration to reduce the implanted depth of the contaminant ions. The implanter may be designed to produce zero energy contamination by extracting the required low energy beams directly from the source, but this inherently runs at much lower beam currents. The other techniques allow for higher currents but do not provide any real time monitoring to ensure that they are effective in preventing energy contamination every time the implanter is run.
In view of the foregoing, it is desired to ensure that the implanter is meeting this contamination requirement at all times while the implantation productivity is maximized.